1. Field of Invention
The present invention relates to a PM type stepping motor, and more specifically, it relates to a PM type stepping motor capable of further reducing vibration and noise.
2. Description of Related Art
A conventional PM type stepping motor proposed by the present applicant is disclosed in Japanese Unexamined Patent Application Publication No. 10-136631. In this conventional PM type stepping motor, to effectively reduce the higher harmonic component of the detent torque, the polar teeth of the yokes of the stator unit are arranged so as to be deviated from positions at equal intervals. That is, when the polar teeth are shifted from positions of equal intervals, the higher harmonic component of the detent torque appearing in principle can be canceled out in the yokes and reduced.
Other conventional PM type stepping motors are disclosed in Japanese Unexamined Patent Application Publication No. 5-252719, Japanese Unexamined Patent Application Publication No. 8-126290, Japanese Unexamined Patent Application Publication No. 3-7050, Japanese Unexamined Patent Application Publication No. 9-233802, etc.
Of these, Japanese Unexamined Patent Application Publication No. 5-252719 discloses a two-phase PM type stepping motor in which a member consisting of a non-magnetic material is provided between first and second phases stacked together in the axial direction.
Japanese Unexamined Patent Application Publication No. 8-126290 discloses a method of joining yokes constituting the stator of a PM type stepping motor.
Japanese Unexamined Patent Application Publication No. 3-7050 discloses a PM type stepping motor in which adjacent polar teeth of the yoke are connected by a bridge.
Further, Japanese Unexamined Patent Application Publication No. 9-233802 discloses a PM type stepping motor which is equipped with a plate spring biasing the rotation shaft of the rotor in one axial direction.
In the PM type stepping motor disclosed in Japanese Unexamined Patent Application Publication No. 10-136631, it is true that, for example, the fourth degree higher harmonic component of the detent torque, can be reduced. However, there is no possibility of both the fourth degree higher harmonic and the second degree higher harmonic being reduced. Further, in some parts, the gap between the polar teeth of the outer yoke and the polar teeth of the inner yoke is extremely small, so that the leakage flux flowing between those polar teeth increases, resulting in a reduction in the torque generated.
On the other hand, careful study made by the present inventors shows that in a PM type stepping motor, the vibration and noise attributable to the axial vibration of the polar teeth due to the electromagnetic force when the motor is driven are not negligible, apart from the vibration and noise attributable to the higher harmonic component of the detent torque periodically generated with respect to the rotation angle of the rotor due to the magneto motive force of the permanent magnet, as has been conventionally reported. However, in the PM type stepping motor disclosed in Japanese Unexamined Patent Application Publication No. 10-136631, nothing is done to reduce the noise such as impact sound attributable to the axial vibration of the polar teeth.
In the PM type stepping motor disclosed in Japanese Unexamined Patent Application Publication No. 5-252719, it is possible to prevent direct collision of the first phase and second phase polar teeth by means of the member provided between the phases. However, exclusively aiming at a reduction in detent torque, the technique does not adopt a construction in which collision of the member interposed between the phases with the polar teeth, and the sound caused by the collision of the member with the polar teeth has resulted in an increase in noise.
In the PM type stepping motor disclosed in Japanese Unexamined Patent Application Publication No. 8-126290, there is provided no joining method as will prevent collision of the motor cover with the polar teeth, and the sound caused by the collision of the cover with the polar teeth has resulted in an increase in noise.
In contrast, in the PM type stepping motor disclosed in Japanese Unexamined Patent Application Publication No. 3-7050, the strength of the base portions of the polar teeth is increased, so that a reduction in the vibration of the polar teeth due to the electromagnetic force when driving the motor can be expected. However, since it adopts a construction in which a bridge is provided, the number of parts is rather large, resulting in an increase in production cost.
Further, in the PM type stepping motor disclosed in Japanese Unexamined Patent Application Publication No. 9-233802, when the reaction force of the electromagnetic force causing the axial vibration of the polar teeth when driving the motor acts on the rotor side, the rotor also axially vibrates, involving elastic deformation of the plate spring and colliding with a member such as the bearing when displaced in the same direction as the biasing direction of the plate spring to thereby generate an impact sound.
The present invention has been made in view of the problems in the conventional techniques. It is one object of the present invention to provide a PM type stepping motor capable of further reducing the higher harmonic component of the detent torque and, further, to provide a PM type stepping motor capable of reducing the noise attributable to the axial vibration of the polar teeth and the rotation shaft due to the electromagnetic force when driving the motor.
According to one exemplary embodiment of the present invention, a PM type stepping motor may consist of a rotor in which N and S magnetic pole pairs of permanent magnets are alternately arranged circumferentially on the outer peripheral surface at a fixed pitch P1, and a stator surrounding the rotor. The stator may consist of a stator unit equipped with an outer yoke and an inner yoke, the outer and inner yokes having comb-teeth-like stator polar teeth in the inner periphery. Each of the outer and inner yokes is equipped with n sets of teeth groups each consisting of m stator polar teeth.
Assuming that the pitch of the stator polar teeth in the teeth group is P2 and the pitch of the teeth groups is P3,
P2xe2x89xa0P1,xe2x80x83xe2x80x83(1)
P3xe2x89xa0mxc2x7P1, andxe2x80x83xe2x80x83(2)
P3xe2x89xa0mxc2x7P2xe2x80x83xe2x80x83(3),
where P1, P2 and P3 are electrical angles, and m and n are integers equal to or larger than 2.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, the teeth groups of the outer and inner yokes may be set such that an arbitrary one of the teeth groups of the outer yoke overlaps circumeferentially only one of the teeth groups of the inner yoke. That is, assuming, for example, that m=3, three polar teeth of an arbitrary teeth group of the outer yoke and three polar teeth of the teeth group of the inner yoke paired therewith are arranged circumferentially as: first of the outer yoke, first of the inner yoke, second of the outer yoke, second of the inner yoke, third of the outer yoke and third of the inner yoke, and do not circumferentially overlap other teeth groups. In the case of a plural phase PM type stepping motor in which a plurality of stator units are stacked together, it suffices that this relationship only holds true in each stator unit.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, the relationship:
P2=P1 {1xc2x1i/u}, andxe2x80x83xe2x80x83(4)
P3=P1 {mxc2x1ixe2x80x2/(nxc2x7xcexdxe2x80x2)}xe2x80x83xe2x80x83(5)
holds true, where u is a positive integer, i is a positive integer which is not a multiple of u, ixe2x80x2 is a positive integer which is not a multiple of n, and xcexdxe2x80x2 is the degree of main higher harmonic reduced.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, the relationship:
P2=P1 {1xe2x88x92i/u}, andxe2x80x83xe2x80x83(6)
P3=P1 {m+ixe2x80x2/(nxc2x7xcexdxe2x80x2)},xe2x80x83xe2x80x83(7)
or the relationship:
P2=P1 {1xe2x88x92i/u}, andxe2x80x83xe2x80x83(8)
P3=P1 {mxe2x88x92ixe2x80x2/(nxc2x7xcexdxe2x80x2)}xe2x80x83xe2x80x83(9)
holds true.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, the relationship:
P2=P1 {1xc2x1i/(mxc2x7xcexd)}, andxe2x80x83xe2x80x83(10)
P3=P1 {mxc2x1ixe2x80x2/(nxc2x7xcexdxe2x80x2)}xe2x80x83xe2x80x83(11)
holds true, where xcexd and xcexdxe2x80x2 are degrees of main higher harmonics reduced, i is a positive integer which is not a multiple of m, and ixe2x80x2 is a positive integer which is not a multiple of n.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, the relationship:
xe2x80x83P2=P1 {1xe2x88x92i/(mxc2x7xcexd)}, andxe2x80x83xe2x80x83(12)
P3=P1 {m+ixe2x80x2/(nxc2x7xcexdxe2x80x2)}xe2x80x83xe2x80x83(13)
or the relationship:
P2=P1 {1xe2x88x92i/(mxc2x7xcexd)}, andxe2x80x83xe2x80x83(14)
P3=P1 {mxe2x88x92ixe2x80x2/(nxc2x7xcexdxe2x80x2)}xe2x80x83xe2x80x83(15)
holds true.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, the stator may have a construction in which a plurality of the stator units are stacked together, and an axial gap may be formed between the base portions of the stator polar teeth of the stator units axially adjacent to each other so that the base portions may not come into contact with each other.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, there may be provided in the outer periphery in the vicinity of the forward end of the stator polar teeth a constraining member formed of a non-magnetic material and for constraining the stator polar teeth.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, the constraining member is a ring.
According to another exemplary embodiment of the present invention, a PM type stepping motor may consist of a rotor in which N and S magnetic pole pairs of permanent magnets are alternately arranged circumferentially on the outer peripheral surface at a fixed pitch, and a stator surrounding the rotor. The stator may consist of a construction in which a plurality of stator units equipped with outer yokes and inner yokes are stacked together, the outer and inner yokes having comb-teeth-like stator polar teeth in the inner periphery. An axial gap may be formed between base portions of the stator polar teeth of axially adjacent stator units so that the base portions do not come into contact with each other.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, there may be provided between the stator units axially adjacent to each other a spacer which is in contact with only the portions other than the base portions of the stator polar teeth of the stator units to thereby form the axial gap.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, the base portions of the stator polar teeth may be deformed so as to be separated from the base portions of the other stator polar teeth axially adjacent thereto to thereby form the axial gap.
Further, according to another exemplary embodiment of the present invention, there is provided a PM type stepping motor which may consist of a rotor in which N and S magnetic pole pairs of permanent magnets are alternately arranged circumferentially on the outer peripheral surface at a fixed pitch, and a stator surrounding the rotor. The stator may consist of a stator unit equipped with outer and inner yokes, the outer and inner yokes having comb-teeth-like stator polar teeth in the inner periphery. There may be provided in the outer periphery in the vicinity of the forward end of the stator polar teeth a constraining member formed of a non-magnetic material and for constraining the stator polar teeth.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, a constraining member for constraining the stator polar teeth of the outer yoke and a constraining member for constraining the stator polar teeth of the inner yoke may be individually provided.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, the width of the constraining member is such as to cover from the vicinity of the forward end of the stator polar teeth of the outer yoke to the vicinity of the forward end of the stator polar teeth of the inner yoke.
According to another exemplary embodiment of the present invention, a PM type stepping motor may consist of a rotor in which N and S magnetic pole pairs of permanent magnets are alternately arranged circumferentially on the outer peripheral surface at a fixed pitch, and a stator surrounding the rotor. The stator may have a construction in which a stator unit equipped with outer and inner yokes is inserted between covers from the axial direction, the outer and inner yokes having comb-teeth-like stator polar teeth in the inner periphery. There may be provided a cover collision preventing structure for preventing collision of the stator polar teeth with the cover.
According to another exemplary embodiment of the present invention, in the above PM type stepping motor, the cover collision preventing structure may have a construction in which the base portions of the stator polar teeth in contact with the cover are connected to the cover.
According to the another exemplary embodiment of the present invention, a PM type stepping motor may consist of a rotor in which N and S magnetic pole pairs of permanent magnets are alternately arranged circumferentially on the outer peripheral surface at a fixed pitch, and a stator surrounding the rotor. The stator may have a stator unit equipped with an outer yoke and an inner yoke, the outer and inner yokes having comb-teeth-like stator polar teeth in the inner periphery. Ribs may be integrally formed by press molding at the base portions of the stator polar teeth.
According to another exemplary embodiment of the present invention, a PM type stepping motor may consist of a rotor in which N and S magnetic pole pairs of permanent magnets are alternately arranged circumferentially on the outer peripheral surface at a fixed pitch, and a stator surrounding the rotor. The rotation shaft of the rotor may be rotatably supported by members on the stator side at two positions axially separated from each other. The stator may have a stator unit equipped with an outer yoke and an inner yoke, the outer and inner yokes having comb-teeth-like stator polar teeth in the inner periphery. The two positions axially separated from each other of the rotation shaft of the rotor may be supported by the rotor side members through the intermediation of elastic members capable of elastic deformation in the axial direction.
In one exemplary embodiment of the invention discussed above, as shown in formula (1), the pitch P2 of the stator polar teeth in the teeth group is different from the pitch P1 of the N and S poles polarized to the rotor, whereby the higher harmonic of the degree corresponding to this deviation is reduced.
It is possible to assume that one teeth group consisting of m stator polar teeth is one polar tooth corresponding to m magnetic pole pairs on the rotor side. Thus, as shown in formula (2), the pitch P3 of the teeth groups is different from the pitch mxc2x7P1 of m magnetic pole pairs, so that the higher harmonic of the degree corresponding to this deviation is reduced.
Further, when the pitch P3 of the teeth groups is m times the pitch P2 of the stator polar teeth in the teeth group, the pitch of the stator polar teeth is fixed over a plurality of teeth groups, and the boundary between teeth groups is lost. Thus, as shown in formula (3), the pitch P3 of the teeth group is different from m times the pitch P2 of the stator polar teeth in the teeth group.
In this way, in this above exemplary embodiment of the invention, higher harmonics of more degrees are reduced, so that the higher harmonics are efficiently reduced, thereby providing a more quiet, smooth rotation.
Further, generally speaking, it is known that when the relationship:
PS=PR {1xc2x11/(mxc2x7xcexd)}xe2x80x83xe2x80x83(16)
holds true between the rotor magnetic pole pitch PR and the stator polar teeth pitch PS, it is possible to reduce a plurality of higher harmonic components including the detent torque xcexd-th degree.
The higher harmonic of each degree is represented by a sine wave of a predetermined period, so that the deviation in stator polar teeth as shown in formula (16) may be i times PR {1/(mxc2x7xcexd)} (I is a positive integer which is not a multiple of m), that is, PR {i/(mxc2x7xcexd)}.
In one exemplary embodiment of the invention discussed above, the pitch P2 of the stator polar teeth in the teeth group is as shown in formula (10), it is possible to reduce a plurality of higher harmonic components including the xcexd-th degree. Further, when the relationship of the pitch P3 of the teeth group and the pitch P1 of the magnetic pole pairs on the rotor side is
P3=mxc2x7P1 {1xc2x1ixe2x80x2/(nxc2x7xcexdxe2x80x2)}xe2x80x83xe2x80x83(17),
a plurality of higher harmonics including the xcexd-th degree are reduced. However, since the higher harmonics generated in the teeth group can be decomposed for m rotor polar teeth, the deviation in pitch of the teeth group with respect to the m magnetic pole pairs on the rotor side may be (1/m).
Thus, from the above formula (17),
P3=mxc2x7P1 {1xc2x1ixe2x80x2/(nxc2x7xcexdxe2x80x2xc2x7m)}=P1 {1xc2x1ixe2x80x2/(nxc2x7xcexdxe2x80x2)},
the above formula (11) is obtained. Accordingly, in this above exemplary embodiment of the present invention, the pitch P3 of the teeth groups is as shown in the above formula (11), so that it is possible to also reduce a plurality of higher harmonics including the xcexdxe2x80x2-th degree.
Further, in one exemplary embodiment of the invention discussed above, assuming that mxc2x7xcexd=u (u is a positive integer) and that i is a positive integer which is not a multiple of u, formula (4) is obtained from formula (10), and the pitch P3 of the teeth groups is as shown in formula (5) (which is the same as formula (11), so that it is possible to reduce a plurality of higher harmonics including the xcexdxe2x80x2-th degree.
Further, in one exemplary embodiment of the invention discussed above, the xe2x80x9cxc2x1xe2x80x9d of the right side of formulas (4) and (5) in this above exemplary embodiment is replaced by a combination of xe2x80x9cxe2x88x92xe2x80x9d and xe2x80x9c+xe2x80x9d (formulas (6) and (7)) or a combination of xe2x80x9cxe2x88x92xe2x80x9d and xe2x80x9cxe2x88x92xe2x80x9d (formulas (8) and (9)), and in another exemplary embodiment, the xe2x80x9cxc2x1xe2x80x9d of the right side of formulas (10) and (11) in yet another exemplary embodiment of the invention is replaced by a combination of xe2x80x9cxe2x88x92xe2x80x9d and xe2x80x9c+xe2x80x9d (formulas (12) and (13)) or a combination ofxe2x80x9cxe2x88x92xe2x80x9d and xe2x80x9cxe2x88x92xe2x80x9d (formulas (14) and (15)).
When the combination in the above exemplary embodiments of the invention is selected, there is no fear of the gap between the rotor polar teeth becoming extremely small, so that it is possible to prevent an increase in the leakage flux flowing between the rotor polar teeth, making it possible to prevent a great reduction in the torque generated.
In one exemplary embodiment of the invention discussed above, the stator has a construction in which a plurality of stator units are stacked together. However, due to the gap formed between the base portions of the stator polar teeth of adjacent stator units, even if the stator polar teeth vibrate in the axial direction due to the electromagnetic force when driving the motor, it is possible to reduce the fear of the base portions of the stator polar teeth colliding with each other. Such a gap can be easily realized.
Further, in one exemplary embodiment of the invention discussed above, there is provided in the outer periphery in the vicinity of the forward end of the stator polar teeth a constraining member formed of a non-magnetic material and constraining the stator polar teeth, so that if electromagnetic force acts on the stator polar teeth when driving the motor, it is possible to prevent deformation of the stator polar teeth. It is not absolutely necessary for the constraining member to be provided at the forward end of the stator polar teeth; it may be provided at a position in the vicinity of the forward end, where it is possible to restrain deformation of the stator polar teeth.
Further, as in one exemplary embodiment of the invention discussed above, the constraining member for constraining the stator polar teeth of the outer yoke and the constraining member for constraining the stator polar teeth of the inner yoke may be individually provided, or the width of the constraining member may be such as to cover from the vicinity of the forward end of the stator polar teeth of the outer yoke to the vicinity of the forward end of the stator polar teeth of the inner yoke, thereby reducing the number of parts. Further, in another exemplary embodiment, a ring may be used as the constraining member, wherein this ring is fitted onto the outer periphery of the stator polar tooth, and wherein the wall thickness of the ring is smaller than the distance between the exciting coil opposed to the stator polar tooth when assembled and the stator polar tooth, whereby assembly can be easily conducted even if the ring is attached to the stator polar tooth.
Further, in one exemplary embodiment of the invention discussed above, there is provided a cover collision preventing structure for preventing collision of the base portions of the stator polar teeth with the cover, so that even if the stator polar teeth vibrate in the axial direction due to the electromagnetic force when driving the motor, it is possible to reduce the fear of the base portions of the stator polar teeth colliding with the cover. Such a cover collision preventing structure can be easily realized by joining the base portions of the stator polar teeth in contact with the cover to the cover (e.g., by soldering).
In one exemplary embodiment of the invention discussed above, ribs are integrally formed in the base portions of the stator polar teeth by press molding, so that even if a force to axially vibrate the stator polar teeth is generated due to the electromagnetic force when driving the motor, it is possible to eliminate or minimize the axial vibration of the stator polar teeth. Thus, it is possible to prevent the stator polar teeth from colliding with other members or minimize the impact if the collision does occur. And, since the ribs are solely integrally formed by press molding, it is possible to reduce the cost as compared with the case in which a bridge is formed as a separate member.
Further, in one exemplary embodiment of the invention discussed above, two portions of the rotation shaft of the rotor axially spaced apart from each other are supported by the stator side member (for example, the cover) through the intermediation of elastic members capable of elastic deformation in the axial direction, so that if a reaction force of the force causing axial vibration of the stator polar teeth due to the electromagnetic force when driving the motor is input, it is possible to reduce the possibility of the rotation shaft colliding with the bearing, etc.